A backlight module is one of the key components for the liquid crystal display (LCD). Since the liquid crystal cannot luminesce itself, the function of the backlight module is to provide the light source, which can provide sufficient brightness and uniform light distribution for image display on the LCD panel. In recent years, since the LCD has been widely applied to an increasing number of electrical products, such as monitors, notebook computers, cameras, personal digital assistants (PDA), mobile phones and projectors, etc., the accompanied demands for backlight modules and other related components are continuously growing.
Please refer to FIG. 1, which is a structural diagram of a backlight module according to the prior art. The backlight module mainly includes the light source 10, the light guide plate 11, the reflector 12, the diffuser 13, the lens sheet 14 and the frame, etc. It is necessary that the light source 10 has the properties of high brightness and long usage duration. Nowadays, the common light sources include the cold cathode fluorescent lamp (CCFL), the hot cathode fluorescent lamp (HCFL), the light-emitting diode (LED), and the electro luminescent (EL) sheet, etc. Among the above, the CCFL is the mainstream in the market these days. The light guide plate 11 is used for directing the scattering direction of the light so that the panel luminance would be increased and the uniformity of the panel luminance would be ensured. The light guide plate 11 is formed through pressing the propylene material into a plate with smooth surface by the injection molding method. Afterward, the particular materials having the properties of high light reflectivity and extremely low light absorption are coated at the bottom surface of the light guide plate 11 for forming numerous diffusion points. Since the diffusion points reflect the light along various angles and directions, the whole light reflectivity of the plate may be influenced. In other words, it is possible that the luminance of the light guide plate 11 can be uniform across the panel by controlling the sizes and densities of the diffusion points. The reflector 12 is used to reflect the leaked light back to the light guide plate 11 so as to improve the utility efficiency of the light. In addition, the function of the diffuser 13 is to distribute the light across the panel more uniformly, and thereby preventing shadows of the diffusion points from being formed on the front side. However, because the directions of the light reflected from the diffuser 13 are too irregular, it is necessary to condense the lights by the lens sheet 14 for achieving a greater brightness at the front side.
In general, the backlight modules can be divided into two groups, the bottom-lighting type and the edge-lighting type, categorized by the positions of the light sources. Nowadays, the light sources for big-size panels are always designed as the bottom-light type. The bottom-lighting backlight module is designed to dispose the lamp tubes just below the module. Since the luminance of the lamp tube is bright and different from that of other parts of the module, it is easy to see the obvious lamp tube from the panel. Further, the obstacles for the pixel design are increased. For avoiding the described flaws, such as the discordant brightness, it is usual to increase the thickness of the light guide plate to a certain thickness. Also, the panel pixels near the lamp tubes are further designed to smaller pixels and have lower pixel-distribution densities. However, in such a way, the thickness and the weight of the backlight modules are always unavoidably increased.